DESCRIPTION Equivalent of up_write(s->s_umount);
deactivate_super(s);, except that it does not unlock it
until it´s all over. As the result, it´s safe to
use to dispose of new superblock on ->get_sb failure
exits - nobody will see the sucker until it´s all
over. Equivalent using up_write + deactivate_super is safe
for that purpose only if superblock is either safe to use or
has NULL ->s_root when we unlock.